Gender differences exist in the neuroendocrine control of GH secretion. The regulatory mechanisms that drive such sex differences are not known, but presumptively are estrogen dependent. We will test if estrogen replacement in postmenopausal women increases GH secretion by modulating the release of GHRH, the endogeneous ligand for GHRP-2 receptor, and/or somatostatin. Insights into the specific mechanisms of estrogen's action will help explicate the coupling between sex steroids and GH axis in health and disease.